There are actually five dwarf planets recognized by the IAU, not three. The five dwarf planets are Pluto, Ceres, Haumea, Eris, and Makemake. Only Ceres and Pluto have been studied in detail. Eris was added to the category since it is bigger than Pluto.
Haumea and Makemake were accepted as dwarf planets because they display absolute magnitude of less than +1 and a minimum diameter of 838km. Those are conditions set by the IAU.
Research does reveal three distinct dwarf planet, which are the asteroid Ceres, Eris, and planet Pluto. By definition, there are many other objects in our solar system that would qualify as being dwarf planets.